This invention relates generally to the field of magnetic data storage devices, and more particularly, but not by way of limitation, to an apparatus and method for controlling correction of microactuator induced hysteresis errors and microactuator assisted seeks of a disc drive actuator.
Disc drives are used as primary data storage devices in modern computer systems and networks. A typical disc drive comprises a head-disc assembly (HDA) which houses mechanical portions of the drive, and a printed circuit board (PCB) mounted to an outer surface of the HDA which supports electronic circuitry used to control the HDA.
Typically, a HDA comprises one or more magnetic discs that are affixed to and rotated by a spindle motor at a constant high speed and an actuator assembly, which supports an array of heads adjacent tracks defined on the disc surfaces. The surface of each disc is a data recording surface divided into a series of generally concentric recording tracks radially spaced across a band having an inner diameter and an outer diameter. The data tracks extend around the disc and store data within the tracks on the disc surfaces in the form of magnetic flux transitions. The flux transitions are induced by an array of transducers, otherwise commonly called read/write heads or heads. Typically, each data track is divided into a number of data sectors that store fixed-size data blocks.
The head includes an interactive element such as a magnetic transducer, which senses the magnetic transitions on a selected data track to read the data stored on the track. Alternatively, the head transmits an electrical signal that induces magnetic transitions on the selected data track to write data to the track. As is known in the art, each read/write head is mounted to a rotary actuator arm and is selectively positionable by the actuator arm over a selected data track of the disc to either read data from or write data to the selected data track. Each head includes a slider assembly with an air-bearing surface that causes the read/write head to fly above the disc surface. The air bearing is developed as a result of load forces applied to the read/write head by a load arm interacting with air currents that are produced by rotation of the disc.
An actuator motor, such as a voice coil motor (VCM), rotates the actuator assembly, and hence the heads, across the disc surfaces. The control circuitry on the PCB includes a read/write channel which interfaces with the heads to transfer data between the tracks and a host computer, and a servo control system which drives the VCM to provide head positional control, based on the information contained in the servo field.
Continued demand for disc drives with ever increasing levels of data storage capacity and data throughput have led disc drive manufacturers to seek ways to increase the storage capacity of each disc surface and improve operating efficiencies of the disc drive. High performance disc drives of the present generation typically achieve areal bit densities measured in several gigabits per square centimeter, Gbits/cm2. Higher recording densities can be achieved by increasing the number of bits stored along each track, and/or by increasing the number of tracks per unit width across each disc. Storing more bits along each track generally requires improvements in the read/write channel electronics to enable the data to be written (and subsequently read) at a correspondingly higher frequency. Providing higher track densities generally requires improvements in the servo control system to enable the heads to be more precisely positioned over the discs. Improved operating efficiencies or throughput performance, for any given bit density, results from reduced cycle times in performing functions or through elimination and/or incorporation of functions internal to the other.
Throughput performance is enhanced during read/write cycles by stabilizing the ability of the servo system to hold the head on track under adverse conditions such as an occurrence of servo field thermal asperity; rotational vibration; resonance of rigid bodies at frequencies sympathetic to the servo frequencies; or components of runout, velocity and acceleration (commonly referred to as RVA) drifting out of tolerance.
To improve on track performance and seek performance for disc drives of higher track densities, disc drive manufacturers are increasingly moving to implement so-called xe2x80x9cmicroactuator motorsxe2x80x9d or xe2x80x9cmicroactuators,xe2x80x9d which are secondary motors suspended by actuator assemblies to provide fine (secondary) positional adjustment of the heads over and above the coarse (primary) positional adjustment provided by the VCMs. A variety of microactuator constructions have been recently proposed in the art, including the use of an inductive rotor/stator arrangement as exemplified by U.S. Pat. No. 5,657,188 issued to Jurgenson et al.; the use of piezoelectric transducers as exemplified by U.S. Pat. No. 6,002,549 issued to Berman et al.; and microelectronic machine (MEM) implementations as discussed in U.S. Pat. No. 5,711,063 issued to Budde et al.
The basic operational concept of an actuator assembly with both primary and secondary motors (also referred to as a xe2x80x9cdual-stagexe2x80x9d actuator) is relatively straightforward; the primary motor is used to bring the selected head within a given range of tracks or to a selected track, after which the secondary motor operates to bring the head over track center of the selected track. While improvements have been made within dual stage actuator technology that dramatically improve head positioning, essentially through the introduction and integration of microactuators into the servo system, challenges remain with maximizing microactuator capability and correcting head position deficiencies stemming from microactuator motor hysteresis.
The present invention provides an apparatus and method of effectuating microactuator assisted seeks and hysteresis corrected head positioning functions for expanding microactuator utility in improving disc drive read/write head placement position relative to a data track.
In accordance with preferred embodiments, a disc drive is provided with a rotatable actuator which supports an array of read/write heads adjacent a corresponding number of recording surfaces in a rotatable disc stack. A servo controller providing a closed loop primary servo control circuit with a closed loop piezo servo circuits generates control inputs for the actuator to carry out track following operations wherein a selected head is caused to follow a corresponding track.
The actuator is characterized as a dual-stage actuator having both a primary actuator motor (VCM), which controllably moves all heads simultaneously, and an array of secondary microactuator motors which controllably move each head individually. Each microactuator preferably has a piezoelectric transducer (PZT) construction and undergoes a dimensional change in response to application of an electric field. Attached to each microactuator is a sensor, such as a strain gauge, which provides a changed electrical resistance in response to a dimensional change imparted to the strain gauge. Hence, actual dimensional change in the microactuator can be directly measured by the sensor.
Control inputs for the motors are determined in relation to an actual position signal for the head, a desired position signal or target track seek request from a control processor and a set of control limits. The control limits are individually determined and loaded into a volatile memory for the selected head at start-up and then as needed for subsequently selected heads throughout the operation of the disc drive. The control limits are initially captured and stored in a head accessible nonvolatile memory location on the disc drive. The control limits are developed by measuring voltage output responses for each microactuator in response to voltage inputs applied to the individual microactuators and the response of each head disc pairing to seek commands.
Each set of control limits comprises the results of the voltage measurements and calculations made on those voltage measurements. The measurements are made during the manufacturing process. The use of a different set of control limits by head enables a servo controller to be adapted for each head/disc combination. That is, as each new head is selected, a new set of control limits appropriate for the new head is loaded into a volatile memory of a servo engine thereby providing accurate, adaptive control.
The primary servo control circuit is a closed loop control circuit that serves, primarily, to control functions of the dual stage actuator during track following and seek requests. A closed loop piezo servo circuit is integrated into the primary servo control circuit and has the ability to operate separately from the primary servo control circuit, having once received input from the primary servo control circuit.
Included within the closed loop piezo servo circuit is a hysteresis correction circuit that monitors and adjusts the position of the head relative to the data track based on an actual physical response of a microactuator to a request for a head positional adjustment. The hysteresis correction circuit monitors the actual physical response of the microactuator to the head position adjustment request and adjusts the input voltage to the microactuator until the actual physical response of the microactuator matches the theoretical physical response of the actuator inherently intended by the original head reposition request. Microactuator hysteresis correction occurs absent servo fields supplied head position data and can occur simultaneous to data transfer functions.
These and various other features and advantages, which characterize the present invention, will be apparent from a reading of the following detailed description and a review of the associated drawings.